Light emitting diodes (LED) are an area of interest in the lighting industry due to energy savings among other desirable attributes. More and more legislation is demanding implementation of such systems to replace typical filament (incandescent) or neon based light structures.
The technology for LED based lighting systems is new and, as such, has constraints which need to be accommodated. For example, most LED luminaries utilize a design that exposes each individual LED to the user that occupies the space the luminaire is illuminating. A single LED luminaire cannot match the output of a single traditional source. Therefore LEDs are typically arranged in an array of between 30 and 200 individual LED's which comprise the acceptable luminaire output.
Each LED in the array is comprised of an electronic semi-conductor which creates an intense point of light source which is generally anisotropic, having an incident beam which disseminates in a direction perpendicular to the plane of the semiconductor substrate. This is quite different in nature than a more traditional incandescent or a florescent lamp which emits in a largely isotropic distribution of light to create what is considered a more even lighting.
LED's are expensive in relation to standard single sources. Most manufacturers have felt that they must optimize every last LED to try to minimize the cost impact and maximize the output. Optics, which can comprise lenses, diffusers, and the like; are used to more evenly distribute the light. These are seen as sources of efficiency loss through transmission loss through lenses or other optics. While this approach may outwardly seem to be the most effective manner to deploy LED luminaires, it creates a significant problem of excessive glare to an occupant directly exposed to the LEDs. Glare can also be referred to as brightness, or in lighting terms as luminance.
Luminance is a photometric measure of the luminous intensity per unit area of light traveling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given angle. The SI unit for luminance is candela per square meter (cd/m2). Another common measurement standard is the United States Customary System (UCS) unit of measure being ft-lamberts. Regardless of units of measure, luminance is measured per unit area of light integrated over an area. Hence, the smaller the area the brighter the surface becomes with the same amount of light transmission.
Many LED manufacturers place a first optic over the top of the bare semi-conductor to control the distribution of the light, designed to achieve a lambertian distribution which is a more even output distribution than that provided by the LED alone. Lambertian Distribution considers the sum of reflections in all directions. When a surface is composed of numerous surfaces such as a polarizer, the overall observed reflection becomes the sum of the individual reflections.
In many cases the first optic is sufficient for distributing the light. But in others, such as structure lighting, a lambertian distribution is ineffective. In these cases a secondary optic is added to the luminaire comprising a lens that is situated over each LED.
The use of second optics is a preferred methodology for achieving directionality rather than changing the primary optics which are more closely integrated into the monolithic silicon. Secondary optics can be created to work in conjunction with the specified conditions. Those skilled in the art will recognize that many combinations of primary and secondary optics can come together to create an equivalent affect, which will be henceforth referenced as a first optic configuration.
The second optic is preferably a bubble refraction design as known to those skilled in the art. The bubble refraction is highly efficient as the primary change in direction of the light is completed through a single light refraction. Additionally reflected light (light that is deflected at the optic interface and did not exit the secondary optic upon first incidence through primary refraction) can be passed through the bubble on the second, third, or even fourth reflection.
In low bay applications, such as parking garage applications, a key concern is eliminating what is known by those in the art as cave effect illumination. Cave effect is where light is distributed directly beneath the fixture while ignoring peripheral areas, creating dark corners and ceilings. Therefore the first optic configuration is directed toward a high angle refraction of the incident beam from each LED in order to create an up-light for illuminating corners and ceilings.
The primary optic configuration alone has shown to be insufficient for creating an aesthetically soothing light distribution suitable for low bay applications. The high intensity of the LED beam coupled with the high angle of refraction of the beam creates a disabling glare for an individual approaching such a low lighting fixture. The lighting guide for professionals (IESNA RP-20) states that the minimum light level must be no less than 1 ft-candle anywhere in the space with a uniformity of no greater than 10:1 (max to min). This means that the luminaire must have a very wide distribution to meet these requirements. This wide distribution means that a large portion of the light emitting from the secondary optic is directed at the same region at a high angle to the luminaire (a generally horizontal plane). Since an LED array comprises many LED's, every LED contributing rays of light into this relatively small high angled area, the overall effect is that the luminaire appears a number of exceedingly bright spots. The brightness can cause significant discomfort to one who views the luminaire in the main beam of light concentration. This discomfort is measured in candela/meter squared, and is quantified by measuring the exitance of light from the luminaire with relation to the angle said light is exiting from the light fixture.
To resolve this high angle brightness, a tertiary optic is added to diffuse the directional light emitted from the first optic configuration to disperse light over a much larger surface area hence reducing the perceived glare from the luminaire. In this instance, disperse can be defined as; “to cause to break up” or “to cause to be spread widely”, and can comprise the mechanisms of diffusion or diffraction. Diffusion can be defined as; “to permit or cause to be spread freely” or “to break up and distribute incident light by reflection”. Diffraction can be defined as: “a modification which light, in passing by the edges of opaque bodies or through narrow slits, or in being reflected from ruled surfaces and in which the rays appear to be deflected.
Adding a tertiary lens in conjunction with the first optic configuration is not straight forward because the light must be diffused or diffracted to integrate the point light sources of the LED in order to appear as a larger, more homogenous, luminary element of lower brightness or intensity than each of the point light sources (main beams) in order to reduce the glare without giving up perceived efficiency or unduly altering the distribution of light.
The lens of the present solution also comprises an element of a thermal management system to conduct waste heat away from the LED array and toward a manifold employing a passive convective heat transfer system. This improvement in heat extraction allows higher driving currents in order to optimize output of the LEDs for a given configuration. Heat generated through operation warms the surrounding air causing it to rise. This is generally referred to as free convection of a fluid. Free convection can be defined as a passive transfer of heat into a fluid (generally the air) causing differences in density of air that thereby causes the flow of air generally in an upward direction or draft. Cooler air from below rises due to the pressure differential and is channeled by a light cover, which also acts as the tertiary lens, toward a manifold where it is concentrated into a laminar flow directed toward the manifold.
Those skilled in the art will recognize that the foregoing explanation is for illustrative purposes and is not limiting in any way upon the principles taught herein. Further, in this scheme it is anticipated that the tertiary lens scheme can comprise a number of configurations. The higher the temperatures the more active the induced convective cooling becomes.
It is therefore an object of the invention to provide a passive heat transfer thermal management system for a light fixture wherein the LED covering provides a means for improved heat transfer and a tertiary optic for light diffusion.
It is therefore an object of the invention to provide a reduced glare from the light fixture.
It is another object of the invention to provide a diffusion of light coming from a high angle of incidence relative to the LED substrate.
It is another object of the invention to provide a heat transfer manifold to aid in convective heat transfer.
It is another object of the invention providing a lighting fixture suited toward low bay applications.
It is another object of the invention a lighting fixture suited toward low bay applications having sufficient up-light for illuminating a parking structure.
It is another object of the invention that this manifold structure be designed to utilize a venturi effect flow to facilitate cooling.
It is another object of the invention to provide a cooling system for inducing convective heat transfer without mechanical means.
It is another objective of the invention to provide a pleasingly aesthetic light fixture.
It is another objective of the invention to provide a lighting fixture which is low maintenance.
It is another objective of the invention that the cooling system will work with luminaries that can illuminate large open spaces and provide adequate illumination to those spaces.